The photo above depicts a globular cluster called M13. A globular cluster refers to a group of stars held in close proximity to one another by gravity. It is assumed that the member stars in a cluster all formed around the same time, from the same gas cloud, and have the same composition. A globular cluster is round, virtually symmetrical, and often contains hundreds of thousands of member stars. Globular clusters, unlike open clusters and associations, only contain very old stars. Since globular clusters are so old, they commonly do not contain any really massive star members any longer. These massive stars begin very bright and blue, but burn out quickly. However, m13 does contain an unusual bright blue star. It is speculated that presence of this unusual star came about by the collision of two smaller, cooler stars. When they collided they formed one massive, hotter, bluer star. We know that it is not a common foreground star as its membership was confirmed by radial velocity measurement. M13 was discovered in 1714 by Edmond Halley. The age of this cluster has been estimated to be between 14 - 24 billion years old. It is found in the constellation of Hercules, giving it the loving title of “Great globular cluster in Hercules.”

In the image of M13 displayed here, we see a number of scattered blue stars. Unlike the particular bright blue member star mentioned above, these blue stars are relatively young. Since stars in a globular cluster all formed around the same time, bright blue member stars should have long since burned out. We can reasonably assume that these stars are not members of M13 but rather foreground stars. We can also see a number of faint red stars in the picture. These are foreground red dwarf stars. Red dwarf stars are relatively cool, causing the reddish color, and have a low-luminosity. The majority of stars pictured in the cluster are red giant stars. Those stars that start out so massive and burn out quickly do not just disappear, they swell up and become red giant stars. Red giant stars are relatively cool but have a high luminosity. As is common to globular clusters, it is clear in the picture that the center of M13 contains a very high concentration of member stars. It is estimated that toward the center of M13 stars are about 500 times more concentrated than in our own solar neighborhood. M13 is 25, 100 light years away. Its linear size is 113.18 light years.